The present invention relates to a phase change memory device, and more particularly, to a phase change memory device using a pnp-BJT which prevents the composition of a phase change layer from being changed and widens the sensing margin of a bit line.
In general, memory devices are largely divided into a volatile RAM (random access memory), which loses inputted information when power is interrupted, and a non-volatile ROM (read-only memory), which can continuously maintain the stored state of inputted information even when power is interrupted. Examples of volatile RAM include, DRAM (dynamic RAM) and SRAM (static RAM), and an example of non-volatile ROM is flash memory such as an EEPROM (electrically erasable and programmable ROM).
DRAM is well known in the art as an excellent memory device; however, the DRAM must have high charge storing capacity requiring the surface area of an electrode to be increased, causing difficulties to accomplish a high level of integration. Further, in flash memory two gates are stacked on each other, and therefore the required operation voltage is high compared to the source voltage. As a result, a separate booster circuit is needed to form the voltage necessary for write and delete operations making it difficult to accomplish a high level of integration.
In order to solve the above problem, efforts in research have been directed towards developing a novel memory device with a simple configuration that is capable of a high level of integration while still retaining the characteristics of a non-volatile memory device. As a result of these efforts, a phase change memory device has recently been disclosed in the art.
In the phase change memory device a phase change occurs in a phase change layer interposed between a lower electrode and an upper electrode. The phase change is from a crystalline state to an amorphous state, and the phase change is due to the flow of current between the lower electrode and the upper electrode. The information stored in a cell is recognized by comparing the resistance of the present state to the medium of the difference between the resistance of the crystalline state and the resistance of the amorphous state.
In the phase change memory device, a CMOS transistor or a PNP-bipolar junction transistor (hereinafter, referred to as a “pnp-BJT”) can be used as a switching element. A phase change memory device with a pnp-BJT has a smaller cell size compared to a phase change memory device with a CMOS transistor. Additionally, since the pnp-BJT has excellent current driving capability, the pnp-BJT can realize a phase change memory device with a high programming current.
A phase change memory device using the pnp-BJT as a switching element has been disclosed in a paper entitled “A 90 nm Phase Change Memory Technology for Stand-Alone Non-Volatile Memory Application” at VLSI 2006.
The phase change memory device adopts a vertical pnp-BJT. In the phase change memory device an emitter region, a base region and a collector region are formed using an ion implantation process. The phase change memory device is formed with a phase change layer and an upper electrode on the emitter region, a bit line that comes into contact with the upper electrode, and a word line that comes into contact with the base region.
However, in the above conventional phase change memory device (which adopts the pnp-BJT as a switching element), since the bit line comes into direct contact with the upper electrode, the upper electrode must be formed so that the upper electrode is thick. When etching an upper electrode material and a phase change material, the etching time is extended due to the increased thickness of the upper electrode. As a result, etch loss is caused in the edge portions of the phase change layer. Due to the etch loss, the composition of the phase change layer will likely change. Accordingly, in the above conventional phase change memory device (with the pnp-BJT), the characteristics of the phase change memory device can be degraded due to the change of the composition of the phase change layer.
Further, in the above conventional phase change memory device (with the pnp-BJT), TiN is the material used as the upper electrode. A TiN upper electrode serves as a factor increasing the resistance of the upper electrode, which has the shape of a bar. Therefore, in the conventional phase change memory device (with the pnp-BJT), since the bar-shaped upper electrode has a high resistance, the sensing margin of the bit line is likely to be degraded.